Heating coils are used to quench components of machines such as automobile bearings.
There are various types of heating coils. For example, a heating coil has a pair of lead plates connected to a power supply such as a high-frequency transformer, lead portions connected to the lead plates respectively, and a coil-shaped head portion connected to the lead portions. Inside the lead portions and the head portion, a flow path is continuously formed to allow cooling water to flow inside the lead portions and the head portion.
According to a first related art, a heating coil has a ring-shaped high-frequency heating coil, a pair of cylindrical power and water lead members connected to the heating coil, and lead plates connected to the power and water lead members respectively, and a high-frequency transformer is connected to the lead plates (see, e.g., JP5-081263U). According to a second related art, a heating coil has a rectangular pipe forming a coil-shaped heating conductor and linear power feeding conductor connected to end portions of the heating conductor, and end portions of the power feeding conductors are connected to a current transformer (see, e.g., JP3408982B2 and JP2540041Y2).
Generally, an electric conductive body has a portion through which an electric current flows easily, and a portion through which an electric current does not flow easily. For example, a direct current is apt to flow through a portion where resistance is small, and an alternating current is apt to flow through a portion where impedance is small (see, e.g., Katsuhiko Hori, “Kogyoyo Denki Kanetsu” (Industrial Electric Heating), The Energy Conservation Center, Japan, 1986).
Regardless of whether it is a direct current or an alternating current, an electric current supplied from a power supply passes through the shortest path in the conductive body.
In the first related art, each of the power and water lead members connected to the ring-shaped high-frequency heating coil has a shape of a circular cylindrical pipe. Thus, when the lead members are arranged in proximity to each other, electric current is concentrated in circular-arc portions of the lead members that are in proximity to each other. That is, the electric current density is high in the circular-arc portions that in the other portions of the lead members, so that efficiency of electric power transmission is low. To increase a quenching amount of a heating target portion from this condition, larger electric current may be supplied to the heating coil. However, due to magnetic force generated in the lead members arranged in mutual proximity, the lead members repel one another, causing an unnecessarily stress to the heating coil. In particular, when a work to be heated is small in inside diameter, the ring-shaped high-frequency heating coil is reduced in size, and a distance between the pair of the lead members is reduced accordingly. Thus, the heating coil is likely to be unduly stressed.
In the second related art, on the other hand, the coil-shaped heating conductor is a rectangular pipe. Thus, when the heating target portion is formed concavely on the inner circumferential surface of a work along a circumferential direction of the work, the heating target portion cannot be heated efficiently. That is, the coil-shaped heating conductors formed by a rectangular pipe is suitable for heating, for example, an axially extending heating target portion of an inner circumferential surface of a work. However, when heating, for example, a concave raceway surface on an inner circumference of an outer ring of a wheel rolling bearing, heat radiated from a linear outer circumferential portion of the rectangular pipe cannot heat the concave raceway surface efficiently.